This invention relates to a connecting method between a waveguide substrate made of electro-optical crystal and an optical fiber and to a reflection preventing film with an ultraviolet light intercepting function for use in putting the method into practice.
In order to build up an optical communication system, various optical apparatus such as an optical modulator and an optical switch are required in addition to basic components such as a light source apparatus and a light receiving apparatus. One of forms of optical apparatus is a waveguide type. An optical apparatus of the waveguide type is normally constituted such that a waveguide is formed on a waveguide substrate made of electro-optical crystal and a phase and so forth of a beam of light confined in the waveguide are controlled. An optical apparatus of the waveguide type has an advantage that, due to its structure, miniaturization can be achieved readily and it can be mass-produced using the planer process or the like as well as another advantage that an electric field can be applied effectively and the power consumption can be reduced significantly. When an optical apparatus of such waveguide type is to be incorporated into a system, the optical fiber as an optical transmission line and a waveguide substrate must necessarily be connected to each other both optically and mechanically. Thus, optimization of a connecting method is being sought.
A conventional connecting method between a waveguide substrate and an optical fiber will be described subsequently with reference to FIGS. 1A and 1B. In those figures, reference numeral 2 denotes a waveguide substrate, 4 a waveguide formed on the waveguide substrate 2, and 6 an optical fiber to be connected. When the waveguide substrate 2 and optical fiber 6 are to be connected to each other, a bonding agent 8 of the ultraviolet light curing type is applied to an end portion of the optical fiber 6 as shown in FIG. 1A, and then high precision positional adjustment is performed so that an end face of a core of the optical fiber 6 and an end face of the waveguide 4 may be opposed to each other and closely contacted with each other, whereafter ultraviolet rays are irradiated in the direction as indicated by arrow marks to cause the bonding agent 8 to cure as seen in FIG. 1B. As a result, the waveguide substrate 2 and optical fiber 6 are connected to each other both optically and mechanically. Where such connecting method is employed, since curing of a bonding agent at a normal room temperature is possible, there is no possibility that the loss characteristics may be varied by thermal stress produced at a connecting portion, as different from an alternative case wherein connection is achieved using a thermosetting bonding agent or solder.
Electro-optical crystal which makes a material of a waveguide substrate is particularly LiNbO.sub.3 or LiTaO.sub.3. However, in case the conventional method is applied to a waveguide substrate made of one of such materials, there is a problem that the waveguide may suffer from optical damage due to an influence of ultraviolet rays which are irradiated in order to cause a bonding agent to cure. Here, optical damage signifies a phenomenon that, when light such as ultraviolet light of a comparatively high intensity is irradiated upon electro-optical crystal of LiNbO.sub.3 or the like, excited electrons drop in, in a process in which electrons are excited from an impurity level in the crystal to a conduction band, to a trap level, and a local electric field is produced in the inside of the crystal so that the refraction index is locally varied by way of an electro-optical effect. Such optical damage is recognized as a reversible phenomenon, and after the irradiation of ultraviolet light is stopped, the variation in refractive index is cancelled. However, since such cancellation does not necessarily proceed quickly, optical damage actually matters when an optical apparatus is used. If the refractive index of a waveguide is locally varied by optical damage in this manner, the loss characteristics of the optical apparatus of the waveguide type may be deteriorated or the operating point (for example, a bias potential in the case of an optical modulator of the waveguide type) may be varied.